There is a widely known Scanning Electron Microscope (SEM). To acquire a two-dimensional image of a scanning region on the surface of a sample, this SEM detects a signal electron generated when the sample is scanned by an irradiation electron beam converged on the sample and displays the signal strength at each irradiation position in synchronization with a scanning signal of the irradiation electron beam.
In an ordinary SEM, the chromatic aberration increases in a low-acceleration region, and a high resolution cannot be achieved. To reduce the chromatic aberration, the deceleration method is effective. By the deceleration method, high-speed passage is made through an objective lens, and irradiation is performed by decelerating the irradiation electron beam directly in front of the sample. Application of the deceleration method decreases the effects of electrical charging and damage caused by electronic irradiation and enables acquisition of information on the sample top surface at a high resolution. Accordingly, a low-deceleration SEM is used for sample surface observation in a variety of fields.
SEM-detected signal electrons are largely divided into backscattered electrons and secondary electrons in terms of the energy released from a sample surface. An electron released to the outside of the sample due to elastic or inelastic scattering of an irradiated irradiation electron beam within the sample is referred to as a Backscattered Electron (BSE). A low-energy signal electron generated in the inelastic scattering process of a backscattered electron and released from the sample surface to the outside of the sample is referred to as a Secondary Electron (SE). FIG. 1 illustrates example energy distribution of secondary electrons (SE) and backscattered electrons (BSE) generated when the energy of an irradiation electron beam is E0. A signal electron with an energy of below 50 eV is commonly referred to as a SE whose generation amount peaks at an energy of several eV. A BSE has a peak at an energy approximately equal to the energy of an irradiation electron. The generation amounts of secondary and backscattered electrons depend on, inter alia, the elements constituting the sample and the energy of an irradiation electron beam. In general, the generation amount of secondary electron is greater than that of backscattered electron.
The generation amount of backscattered electron depends on the average atomic number, density, and crystallinity of a sample at the irradiating position of an irradiation electron beam. When a secondary electron is not detected and only a backscattered electron is detected in an SEM image, a contrast that reflects sample composition and crystal orientation differences can be obtained. To the contrary, as a secondary electron is generated on the sample surface, a contrast reflecting sample unevenness and electric potential differences can be acquired. By separately detecting a secondary electron and a backscattered, electron, different types of sample information can be acquired. There exist many types of SEMs installed with a plurality of detectors that are directed to separately acquiring various types of sample information acquired from SEM observation.
In particular, during SEM observation in a low-acceleration region, it is necessary to reduce the aberration generated when an irradiation electron beam passes through an objective lens so as to achieve a high resolution. The distance between the sample and the top end portion of an objective lens (Working Distance: WD) needs to be set to a small value, i.e., below several mm. When the deceleration method is employed under these observation conditions, many signal electrons pass through the objective lens while being accelerated. A detector directed to detecting a signal electron should be installed closer to the electron source side than to the objective lens. In an SEM provided with the above type of detection system, separately obtaining different types of sample information using different detectors tends to be considered important.